A pump assembly of the general type described above is disclosed in U.S. Pat. No. 5,094,596 to Erwin, Cavanaugh and Hetherington. This patent includes a description of a pump assembly having a piston stroke shorter than the pumping chamber, where the piston is maintained in continuous engagement with a piston seal. The cylinder within which the piston operates is provided with in-line inlet and outlet check valves to create a single pumping chamber for each pump, as opposed to separate intake and output chambers. Each pumping chamber is of substantially uniform diameter from the piston seal to the end wall of the chamber, and the inlet and outlet check valves are axially aligned with one another parallel to and immediately adjacent the end wall of the pumping chamber, so that fluid flow through the chamber is essentially a straight line of flow diametrically across the chamber without angular transitions. To maintain an essentially straight line path of flow, the reciprocatory stroke of the pump piston is maintained quite short. A short stroke of the piston is possible because the piston is not withdrawn from its seal, and the suction force of the piston is transmitted directly to the associated inlet check valve.
The short stroke in combination with the aforementioned pump assembly produced significant increases in pumping capacity, with less down-time and service requirements. In addition, in the event of failure of the seal assembly between the motor and the piston rod of either pump unit, motor fluid leaking from the motor through the seal assembly was prevented from entering into the path of flow of the fluent material in the pumping chamber and thus did not render the pump inoperable--the motor fluid was simply vented to the atmosphere harmlessly. This feature of the pump of the prior art facilitated operation of the motor by means of hydraulic fluid, since hydraulic fluid leaking from the actuator would not result in contamination of the fluent material and could likewise be drained away harmlessly.
However, although the aforementioned pump assembly resulted in significant improvements in the operation of the unit and in a significant decrease in the power required for a given amount of work, or conversely, a significant increase in the pumping capacity for a given energy input, potential drawbacks were visualized in the possible event of unrecovered leakage of certain fluent materials and the passage of such fluent materials to the environment.
The prior pump assembly was designed so as to accommodate leakage of the fluent material past the piston seal and venting of the fluent material to atmosphere, especially as piston seal wear became more pronounced. The area rearwardly of or behind the piston was configured as a static chamber vented to the atmosphere so as to avoid any fluent material impediment to, and consequent wasteful consumption of power on, the return or suction stroke of the piston. Fluent material thus collected in the static chamber could be vented away from the static chamber for disposal or recirculation back to the fluent material source of supply. Such venting of the fluent materials also allowed detection of leakage past the piston seal. By providing such a vent rearwardly of the piston, fluid leaking past the piston seal was readily observable, such that the amount of leakage would indicate to an operator an appropriate time to replace the seal. Venting of this space, however, also allowed the escape of the fluent materials.
When pumping expensive fluent materials (i.e., certain coating materials based on polymers) and environmentally controlled materials (i.e., coating materials containing highly volatile organic compounds (or so-called "VOCs") such as found in many solvents), it is important that the material not be wasted and that the VOCs not escape to the ambient atmosphere and thus the environment. In the case of the prior art, if the pump piston seal leaked, these disadvantages could occur.
Also, should corrosive solvents pass by the piston seal into the static chamber, a potential could be created for failure of the seal between the static chamber and the actuator, which could in some cases allow fluent material to continue into the actuator and intermingle with the motor fluid, such as air or hydraulic fluid. This could be especially problematic in situations where the fluent material is pre-charged prior to delivery to the pumping chamber. Seal failure could therefore result in damage to the pump, waste of expensive fluent materials, and passage to the atmosphere of environmentally controlled substances.